This invention relates to low level light sources for lifting the veil of darkness in at least a portion of the light spectrum and, in particular, to an electroluminescent (EL) source of infrared radiation.
Low level light sources are used wherever there is desired sufficient light for mobility but not acuity, such as night lights and emergency lights, or where a light source is viewed directly rather than used as a source of illumination, such as marker lights. A popular source of such lighting is chemiluminescent sticks, in which two or more chemicals are mixed to produce a photochemical reaction. The container for the mixed chemicals acts as a tubular lamp. Problems with chemiluminescent sticks include low luminance, short life (defined as the time to half of initial luminance), sensitivity to jarring, disposal of materials, and the inability to turn the light off after the reaction is started. Chemiluminescent sticks typically have a life of approximately twenty minutes but will glow weakly for several hours.
An alternative to the chemiluminescent stick is a tubular electroluminescent lamp such as disclosed in U.S. Pat. No. 6,075,322 (Pauly). An EL lamp in the form of a flat sheet is rolled into a cylinder with the luminous side facing outward and stored in a transparent tube containing batteries and an inverter for driving the lamp. The EL lamp includes a dielectric layer between two conductive electrodes, one of which is transparent. The dielectric layer includes a phosphor powder or there is a separate layer of phosphor powder adjacent the dielectric layer. The phosphor powder emits light in the presence of a strong electric field, using very little current. An EL lamp requires high voltage, alternating current but consumes very little power, even including the current drawn by an inverter for driving an EL lamp.
It has long been known in the art to xe2x80x9ccascadexe2x80x9d phosphors, i.e. to use the light emitted by one phosphor to stimulate another phosphor or other material to emit light at a longer wavelength; e.g. see U.S. Pat. No. 3,052,810 (Mash). It is also known to doubly cascade light emitting materials. U.S. Pat. No. 6,023,371 (Onitsuka et al.) discloses an EL lamp that emits blue light coated with a layer containing fluorescent dye and fluorescent pigment. In one example, the pigment absorbs blue light and emits green light, while the dye absorbs green light and emits red light.
It is known in the art to produce infrared light from an EL lamp. In U.S. Pat. No. 4,857,416 (Kreiling et al.) a cascading fluorescent dye produces light with an infrared component. Visible light is filtered out, leaving the infrared light. The patent relies on absorbtion rather than emission, which necessarily means that relatively little infrared light is produced.
There is a need in the art for small lamps that produce infrared radiation but not visible radiation. It is often desired to mark an area or light a room without everyone being aware of the presence of a lamp. For example, law enforcement officers might want to illuminate a room with infrared light prior to entering the room, enabling the room to be scanned with a xe2x80x9cnight visionxe2x80x9d camera or rifle scope. For temporary markers, it is critical that the marker be able to be shut off when not needed or wanted. So-called xe2x80x9cblackxe2x80x9d lights cannot be used because such lamps emit ultraviolet radiation, which is harmful to the human eye.
In view of the foregoing, it is therefore an object of the invention to provide an EL lamp that emits sufficient infrared light to be used as a source of illumination.
Another object of the invention is to provide an EL lamp that emits infrared light in a band that matches the sensitivity of night vision devices.
A further object of the invention is to provide an infrared light source that can be turned on and off at will.
Another object of the invention is to provide an infrared light source that has a life of several hundred hours.
A further object of the invention is to provide an EL lamp that can be a flat light source or a three dimensional light source.
Another object of the invention is to provide a portable infrared light source that is insensitive to jarring.
The foregoing objects are achieved in this invention wherein at least one layer of cascading material converts the light emitted by an EL lamp into infrared light. The EL lamp is preferably supported within in a container transparent to at least infrared light. At least one battery and an inverter provide power for the lamp in portable applications and the container also encloses the battery and inverter. The lamp can be turned on or off by means of a switch interrupting current from a battery or to the lamp.